"What?" The Doctor asked. "Oh, right. The glasses." It was always tricky, asking the Doctor to explain himself. If he were particularly pleased with himself (or particularly angry), you'd get more than an earful. If he were busy, you'd get nothing. And, if he thought you wouldn't understand the long version, he'd answer with a quick (and unintentionally patronizing) gloss-over.
Fred got the long version. "It's a perception filter. The filter projects a mild telepathic field; it convinces you that you want to see something other than what's really there. Not foolproof, but it's good enough."
He held up his pair of sunglasses, about a foot from his face, and looked through the tinted lenses. "Glasses off, everything's normal. Glasses on, people generally can't be bothered to look at you. It's how we're going to get back in, but I'll warn you - make enough of a fuss and you'll be seen, perception filter or no. You're not invisible."
As for the City natives, well. He filed it away for later investigation, once they'd finished with Arkham and everyone else was free. The Doctor pushed Fred's pair of glasses across the table. "Are you up to a little sneaking-about?"